Tribology is a science of friction, wear, and lubrication on friction surfaces. Mechanical testing machines and tribometers are used for testing parts, materials, coatings, lubricants, etc. for evaluating mechanical properties, durability, wear resistance, lubricity, etc. of tested specimens. In such mechanical testing machines tested specimens are subject to an axial (usually vertical) compression with simultaneous rotation around the axis of compression, thus performing so-called twist-compression, thrust washer, disc-on-disc, ring-on-disc, four-ball, drilling or tapping torque tests.
Various types of sensing devices based on strain-gauges and other techniques for monitoring and controlling the magnitude of applied forces and measuring torques are known in the art, such as the load cells and torque sensors manufactured and supplied by Measurement Specialties, Transducer Techniques, Omega Engineering, Interface Inc., and others. In order to measure simultaneously both the force and the torque applied to a workpiece a load cell and a torque sensor can be assembled together, one on top of another. An example of such an arrangement is shown schematically in FIG. 1, where a sensor assembly, which in general is designated by reference numeral 10 comprises a torque sensitive element or a torque sensor 11 attached to a force sensitive element or a load cell 12. This sensor assembly is used on a testing machine, which in FIG. 1 is represented by a sensor mounting plate 15. An upper test material specimen or probe 20 fixed in a holder 22 attached to the sensor assembly. In a course of a test the upper specimen 20 is centered and brought in contact with a lower disc-like test material specimen 24 fixed on a rotary drive coupled to a lower platform of the testing machine (not shown), which performs rotary motion in the direction indicated by arrow R, while being in contact with the stationary upper specimen 20. A loading force Fz applied to the upper specimen is monitored by the load cell 12, while a reaction torque Tz is measured by the torque sensor 11.
In another application, a force-torque sensor assembly 10 can be attached to the lower platform of the testing machine and the lower disc specimen 24 can be fixed in the holder 22, while the rotary drive with the upper specimen 20 can be attached to the mounting plate 15. In that case the upper specimen 20 is rotating while applying the loading force Fz on the stationary lower specimen 24.
Such a design not only results in increased the testing setup total length, but also can cause a significant mutual influence or cross-talk between the force and the torque sensitive elements and measurement channels, i.e., applied load Fz can affect the torque sensor 11 and vice versa, the torque Tz applied to the specimen 20 can affect the load cell 12, since the load cells are usually designed to withstand uniaxial tensile or compression forces and may not have enough immunity against a twisting torque. In turn, the torque sensors may react not only on a twisting torque but also have a significant sensitivity to applied axial load.
In order to reduce dimensions of measuring devices and of testing machines various multi-component sensing devices and/or transducers can be employed, such as the ones supplied, for example, by ATI Industrial Automation, Schunk, Interface, and some other manufacturers. Such sensing devices usually comprise a multi-beam elastic member with deformation-sensitive elements, which react on complex deformations caused by the forces and torques applied to the elastic member. The resulting output signal is being processed using a compliance matrix technique and the applied force and torque components resolved by using special computation algorithms.
For example, U.S. Pat. No. 9,448,128 issued in 2016 to B. Kim, at al. (schematically shown in FIG. 2), U.S. Pat. No. 8,776,616 issued in 2014 to P. Szasz, at al., U.S. Pat. No. 8,671,780 issued in 2014 to S. Kwom, at al., describe various multi-axial force-torque sensors, each of them typically comprising a central hub 120 (FIG. 2), an outer rim 121, a plurality of elastic members or beams 122 each having one side connected to the hub and the other side connected to the outer rim, and strain gauges 124 placed in defined areas on the beams to measure strain, from which forces and torques are calculated.
U.S. Pat. No. 7,047,826 issued in 2006 to M. Peshkin, describes a sensor (shown in FIG. 3) for measuring force and/or torque about a single axis or multiple axes, comprising a first inner member 220, a second outer member 230, an elastic member or a flexure 240 connecting the first member 220 and the second member 230, a handle 210 connected to the first inner member 220, mounting holes 250, and protective pins 260. Applying a force to the handle 210 moves the inner member 220 relative to the outer member 230 as controlled by the flexure 240. The amount or degree of the movement of the inner member 220 relative to the outer member 230 detected as a function of applied force and/or torque.
Performance and accuracy of such force-torque sensors may be affected significantly by structural errors due to the shape of elastic members and inaccuracy of the sensor body, as well as by signal processing errors, both resulting in a significant inter-channel cross-talk.
Another disadvantage of the multi-component force-torque sensors based on multi-beam elastic elements is the fact that such sensors have approximately similar sensitivity to all of the applied force and torque components, thus making it difficult to produce a sensor with significantly different working ranges for a loading force and a reaction torque.
Yet another disadvantage of the multi-component force-torque sensors based on multi-beam elastic elements is a complexity of their manufacturing processes and related to it their higher cost, and a fact that in case of a failure of a single beam the entire sensor becomes non-operational and non-repairable.
Thus, a need exists for a compact, reliable, and cost-effective multi-component force-torque sensor with increased mechanical stability and reduced cross-talk between the measured components.